Electrode foil for electrolytic capacitor and method for manufacturing the same

ABSTRACT

A manufacturing method including: forming a boehmite film on a roughened surface of an aluminum foil; forming a particulate gel of a valve metal oxide on the surface of the aluminum foil on which the boehmite film is formed; then carrying out heat treatment for forming particulate valve metal oxide; and anodizing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode foil for an electrolytic capacitor used for various electronic equipments and a method of manufacturing the same.

2. Background Art

In recent years, as electronic equipment has become smaller, been digitized and had higher reliability, users have strongly demanded smaller electrolytic capacitors. Thus, an anode foil and a cathode foil used for an aluminum electrolytic capacitor have been required to have larger electrostatic capacitance per unit area.

Hereinafter, a typical method of manufacturing an electrolytic capacitor is described. An anode foil having a dielectric oxide film formed on the surface thereof is produced by anodizing an aluminum foil that has an increased effective surface area by etching. Furthermore, a cathode foil that has an increased effective surface area by etching is produced. By winding the anode foil and the cathode foil via a separator, a capacitor element is configured. The capacitor element is impregnated with electrolytic solution for driving, and then accommodated in a metallic case. An opening portion of the metallic case is sealed with a sealing material. Thus, an electrolytic capacitor is manufactured.

In order to increase electrostatic capacitance of the anode foil and the cathode foil per unit area, in general, the effective surface area is increased by chemical or electrochemical etching in an aqueous solution of hydrochloric acid added with acid for forming a film, for example, sulfuric acid, nitric acid, phosphoric acid, and oxalic acid, etc. By changing treatment conditions such as compositions of the electrolytic solution, current density, and the like, properties such as electrostatic capacitance, mechanical strength, and the like, of the anode foil and the cathode foil are improved.

The electrostatic capacitance may be increased by carrying out pretreatment after etching and before anodizing. In the pretreatment before the anodizing, hydrate film treatment is carried out. By forming a hydroxide film on the etched surface of the aluminum foil, the amount of electricity to be used for anodizing is saved, thus enabling electrostatic capacitance to be increased. Usually, a method of dipping an aluminum foil into high temperature pure water is carried out.

However, even if hydrate film treatment before anodizing is carried out in a high temperature (90° C. or higher) pure water added with amine, electrostatic capacitance cannot be increased.

Furthermore, for example, Japanese Patent Unexamined Publication No. 08-167543 describes the following technique for improving electrostatic capacitance by a dielectric oxide film. That is to say, this publication describes that by anodizing by mixing dielectric fine particles having high dielectric constant, for example, TiO₂, PbZrO₂, BaTiO₃, etc. into electrolytic solution, a dielectric oxide film mixed with dielectric fine particles can be formed, thus enabling the electrostatic capacitance to be enhanced.

However, in the electrode foil in which a dielectric oxide film mixed with dielectric fine particles is formed, it is difficult to uniformly mix the dielectric fine particles into the electrolytic solution. Therefore, dielectric fine particles cannot be uniformly dispersed in the dielectric oxide film, so that electrostatic capacitance cannot be increased.

Furthermore, for example, Japanese Patent Unexamined Publication No. 2003-224036 describes a technique of forming valve metal oxide (excluding aluminum oxide in this case) on a barrier type aluminum oxide film and then anodizing in electrolytic solution again. That is to say, the publication describes that by forming a dielectric oxide film composed of a barrier type aluminum oxide film and a mixed layer of aluminum oxide and valve metal oxide, it is possible to obtain an aluminum anode foil in which electrostatic capacitance is improved and leakage current is reduced.

However, when the mixed layer of aluminum oxide and valve metal oxide is formed, since a barrier type aluminum oxide film layer is basically present, the dielectric constant depends upon the value of Al₂O₃. Furthermore, when the mixed layer is formed by a sol-gel method, sol/gel particles makes it difficult to obtain a generally called low voltage foil (withstanding voltage: 30V or lower).

SUMMARY OF THE INVENTION

A method of manufacturing an electrode foil for an electrolytic capacitor of the present invention includes: forming a boehmite film on a roughened surface of an aluminum foil; forming a particulate gel of a valve metal on the surface of the aluminum foil on which the boehmite film is formed; then carrying out heat treatment for forming particulate valve metal oxide; and anodizing the aluminum foil.

Furthermore, an electrode foil for an electrolytic capacitor of the present invention includes particulate valve metal oxide formed on a roughened surface of an aluminum foil, and a dielectric oxide film layer formed in a way in which it covers the particulate valve metal oxide.

The method of manufacturing an electrode foil for an electrolytic capacitor of the present invention provides a less defective dielectric oxide film and enables high electrostatic capacitance even when usual anodizing is carried out and can provide an electrode foil manufactured by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an electrode foil for an electrolytic capacitor according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have found from experiment that when treatment for producing a hydrate firm, before anodizing, is carried out by using pure water dissolving amine at high temperature (90° C. or higher), an etched surface of aluminum is dissolved, so that electrostatic capacitance is lowered. The electrostatic capacitance is not increased even when the withstanding voltage is 150V or less. It is thought that treatment in high temperature pure water makes the hydrate film thick and that the hydrate film remains even after anodic oxidation is carried out. The thick hydrate film may affect badly to the increase in the electrostatic capacitance.

Thus, a method of manufacturing an electrode foil for an electrolytic capacitor of the present invention includes previously forming of a boehmite film on a roughened surface of aluminum foil. The forming of a boehmite film can enhance the holding ability of particulate gel and enables a particulate valve metal oxide layer to be formed densely and uniformly in the following heat treatment. The forming of a boehmite film has also a function of enhancing the anode efficiency when the anodic oxidation is carried out.

By dipping the roughened aluminum foil into a weak alkaline aqueous solution and then subjecting it to heat treatment, a very thin boehmite film can be formed. In particular, it is preferable to use a weak alkaline aqueous solution at pH 8 to 10. Unlike a hydrate film formed by.the pure water boiling treatment, the boehmite film of the present invention does not cause a problem that the boehmite film remains on the surface of the dielectric oxide film when aluminum is anodized and affects electrostatic capacitance and leakage current property.

Furthermore, the particulate valve metal oxide formed on the boehmite film serves as a buffer layer when the anodizing is carried out, making the growth rate of the dielectric oxide film slow during anodizing. Consequently, it is possible to form a dielectric oxide layer that is thinner, denser and more uniform than a dielectric oxide layer obtained by conventional anodizing.

Since a part of the particulate valve metal oxide is bonded to the dielectric oxide film to form composite oxide, it is possible to obtain an electrode foil having a small leakage current and high electrostatic capacitance.

In the thus obtained electrode foil for an electrolytic capacitor, by forming particulate valve metal oxide, a dielectric oxide layer formed by anodizing becomes a dense and uniform layer. Furthermore, since it is possible to realize equal withstanding voltage with a thinner dielectric oxide layer as compared with a conventional dielectric oxide layer, the thickness per 1V of withstanding voltage becomes thinner than usual. Therefore, when comparison is carried out with withstanding voltage equalized, the obtained electrostatic capacitance of the dielectric oxide layer of the present invention becomes higher than that of a conventional dielectric oxide layer.

Hereinafter, embodiments of the present invention are described.

An electrode foil for an electrolytic capacitor of the present invention has a configuration as shown in FIG. 1. As shown in FIG. 1, on a surface of aluminum foil 11, particulate valve metal oxide 12 is formed and amorphous dielectric oxide layer 13 is formed in such a manner in which it covers valve metal oxide 12. FIG. 1 shows the cross sectional view of valve metal oxide 12 as a circular form. The form of valve metal oxide 12 is not limited to a spherical shape, and a rod shape, a tabular shape and a feathery shape, for example, can be possible.

Next, a method of manufacturing this electrode foil for an electrolytic capacitor is described.

Aluminum foil 11 to be used has a thickness of 100 μm and purity of 99.98% or higher. Aluminum foil 11 is subjected to pretreatment if necessary.

Next, electrolytic etching is carried out by dipping aluminum foil 11 into aqueous electrolytic solution including acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and the like. The etching is carried out to form a roughened surface of aluminum foil 11. Thereafter, dechlorination is carried out and then a boehmite film is formed on the roughened surface of aluminum foil 11.

This boehmite film can be formed by dipping aluminum foil 11 into a weak alkaline aqueous solution, followed by heat treatment. In particular, it is preferable that aluminum foil 11 is dipped in a weak alkaline aqueous solution at pH 8 to 10.

Since the thus formed boehmite film is easily oxygen-bonded to particulate gel formed thereafter, it can enhance the holding ability of particulate gel. Furthermore, since this boehmite film allows dielectric oxide layer 13 formed in the following anodizing to be generated easily, it enhances the formation efficiency in the anodizing and allows dielectric oxide layer 13 to be formed densely and uniformly. Since the boehmite film is incorporated into dielectric oxide layer 13 through the anodizing, it does not exist after the anodic oxidation is carried out.

Unlike the hydrate film formed by the pure water boiling treatment, this boehmite film can be a very thin boehmite film by treatment with a weak alkaline aqueous solution at pH 8 to 10 at a liquid temperature of 50° C. to 70° C., followed by heat treatment. Consequently, it does not cause a problem that the boehmite film remains on the surface of the dielectric oxide film that was anodized and affects electrostatic capacitance and leakage current property.

The weak alkaline aqueous solution at pH 8 to 10 can be prepared from an aqueous solution of ammonia, ethylamine, and triethanolamine, etc.

Next, on the surface of aluminum foil 11 on which a boehmite film is previously formed, particulate gel of valve metal is formed. This particulate gel is oxide of at least one metal selected from Si, Ti, Zr, Nb, Al and Ta. Aluminum foil 11 on which a boehmite film is formed is impregnated with sol solutions thereof or an alkoxide solution.

A sol solution is a solution containing particles having an outer diameter in a range of 1 to 200 nm. More preferably, a sol solution containing particles having an outer diameter in a range of 5 to 50 nm is used. Thus, the inside of roughened aluminum foil can be impregnated with particles.

Subsequently, by subjecting aluminum foil 11 provided with particulate gel to heat treatment, particulate valve metal oxide 12 is formed on the surface of aluminum foil 11. The outer diameter of the formed oxide is substantially equal to the outer diameter of the particulate gel. Therefore, it is preferable that the outer diameter of the metal oxide particles is preferably in a range of 5 to 50 nm.

Next, anodizing is carried out by dipping aluminum foil 11 on which particulate valve metal oxide 12 is formed into an aqueous solution of inorganic acid or organic carboxylic acid, thereby dielectric oxide layer 13 is formed. At this time, particulate valve metal oxide formed on the boehmite film serves as a buffer layer, making the growth rate of the dielectric oxide film slow when the formation is carried out. Consequently, it is possible to form a dielectric oxide layer that is thinner, denser and more uniform than a dielectric oxide layer obtained by conventional anodizing.

Hereinafter, the Embodiments are described in more detail in accordance with more specific examples.

EXAMPLE 1

Firstly, pretreatment is carried out by dipping an aluminum foil having a thickness of 100 μm and a purity of 99.98% into an aqueous solution having a phosphoric acid concentration of 1.0 wt % at 90° C. for 60 seconds.

Then, this aluminum foil is subjected to etching treatment so as to roughen the surface rough.

The etching treatment was carried out by dipping the aluminum foil into an etching bath filled with electrolytic solution (aluminum concentration in the electrolytic solution was adjusted to 0.1 wt %) at 30° C. which included hydrochloric acid: 5 wt %, aluminum chloride: 2 wt %, sulfuric acid: 0.1 wt %, phosphoric acid: 0.5 wt %, and nitric acid: 0.2 wt %, and then applying alternating voltage (frequency of 35 Hz, for 100 seconds) to a pair of electrode plates.

After this etching treatment, the aluminum foil was dipped into 10% sulfuric acid aqueous solution to carry out dechlorination, dipped into an aqueous ammonia solution at pH 8 (temperature: 60° C.) for one minute, and then subjected to heat treatment at 350° C. Thus, a boehmite film was formed.

Then, the aluminum foil on which the boehmite film was formed was dipped in a silica sol involving particles of which particle size is 4-6 μm (trade name “SNOW TEX XS”, NISSAN CHEMICAL INDUSTRIES LTD.) and subjected to heat treatment at 300° C. for two minutes. This operation was repeated three times to form particulate Si oxide on the surface of the boehmite film.

Then, aluminum foil 11 on which particulate Si oxide 12 was formed was dipped into an aqueous solution of inorganic acid or ammonium adipate (concentration: 5%) and anodic oxidation (applied voltage: 22V) was carried out to form dielectric oxide layer 13. Thus, an electrode foil for an electrolytic capacitor was produced.

EXAMPLE 2

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that hydrogen ion concentration when the boehmite film was formed was adjusted to pH 9.0.

EXAMPLE 3

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that hydrogen ion concentration when the boehmite film was formed was adjusted to pH 10.0.

EXAMPLE 4

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that hydrogen ion concentration when the boehmite film was formed was adjusted to pH 7.5.

EXAMPLE 5

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that hydrogen ion concentration when the boehmite film was formed was adjusted to pH 10.5.

EXAMPLE 6

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that an aqueous solution of triethanolamine (TEA) at pH 8.5 (liquid temperature: 55° C.) was used instead of aqueous ammonia for forming the boehmite film.

EXAMPLE 7

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that alumina sol (trade name “Alumina Sol 100,” NISSAN CHEMICAL INDUSTRIES LTD.) was used instead of silica sol.

EXAMPLE 8

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that titanium oxide sol produced by dissolving titanium tetraethoxide (Ti(OC₂H₅)₄) in an ethanol solution and adding H₂O and HCl thereto was used instead of silica sol in Example 1.

EXAMPLE 9

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that niobium oxide sol produced by dissolving niobium tetraethoxide (Nb(OC₂H₅)₄) in an ethanol solution and adding H₂O and HCl thereto was used instead of silica sol in Example 1.

EXAMPLE 10

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that zirconium sol produced using isopropoxide (Zr(O-i-C₃H₇)₄) dissolved in ethanol was used instead of silica sol in Example 1.

Comparative Example 1

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that the forming of a boehmite film and the forming particulate valve metal oxide were not carried out.

Comparative Example 2

An electrode foil for an electrolytic capacitor was produced in the same manner as Example 1 except that the forming of particulate valve metal oxide was not carried out. TABLE 1 Forming characteristics condition of bulb With- Boehmite film Metal standing Capa- Leakage Aqueous oxide Voltage citance current pH solution particle (V) (μF) (μA) Example pH 8.0 Aqueous Si 23.8 114 0.048 1 ammonia oxide Example pH 9.0 Aqueous Si 23.7 112 0.049 2 Ammonia oxide Example pH 10.0 Aqueous Si 23.3 110 0.053 3 Ammonia oxide Example pH 7.5 Aqueous Si 22.7 107 0.069 4 Ammonia oxide Example pH 10.5 Aqueous Si 23.2 106 0.064 5 Ammonia oxide Example pH 8.5 Aqueous Si 24.1 118 0.045 6 TEA oxide Example pH 8.0 Aqueous Al 23.8 115 0.047 7 Ammonia oxide Example pH 8.0 Aqueous Ti 23.7 114 0.048 8 Ammonia oxide Example pH 8.0 Aqueous Nb 23.6 116 0.047 9 Ammonia oxide Example pH 8.0 Aqueous Zr 23.6 113 0.045 10 Ammonia oxide Compar- None None 22.2 100 0.112 ative 1 Compar- pH 8.0 Aqueous None 22.5 103 0.104 ative 2 ammonia

Properties of the electrode foils for an electrolytic capacitor of Examples 1 to 10 and Comparative Examples 1 and 2 were evaluated. The results are shown in Table 1. Note here that for evaluation of properties, withstanding voltage, electrostatic capacitance and leakage current of the films were measured based on EIAJ regulations (EIAJ RC-2364A standard). Herein, the electrostatic capacitance of Comparative Example 1 is normalized to be 100 μF.

In the electrode foils for an electrolytic capacitor of Examples 1 to 10, a boehmite film and particulate valve metal oxide are formed beforehand and thereafter anodizing is carried out. On the contrary, in the electrode foil for an electrolytic capacitor of Comparative Example 1, these are not formed. As shown in Table 1, the electrode foils for an electrolytic capacitor of Examples 1 to 10 can improve the film withstanding voltage by 2 to 9%, electrostatic capacitance by 6 to 18%, and leakage current by 40 to 60% as compared with the electrode foil for an electrolytic capacitor of Comparative Example 1. These values have been dramatically improved as compared with values that can be obtained by optimizing etching conditions and formation conditions.

The electrode foil with the hydrogen ion concentration (pH) of a weak alkaline aqueous solution constituting a boehmite film of less than pH 8 (Example 4) shows that film withstanding voltage and electrostatic capacitance tends to be lowered. On the other hand, the electrode foil with the hydrogen ion concentration (pH) of more than pH 10 (Example 5) shows that electrostatic capacitance tends to be lowered.

Electrode foils using particulate valve metal oxide including any one of Si oxide (Example 1), Al oxide (Example 7), Ti oxide (Example 8), Nb oxide (Example 9), and Zr oxide (Example 10) can provide higher properties than the electrode foil of Comparative Example 1.

When the electrode foil for an electrolytic capacitor of Comparative Example 2 in which a boehmite film is formed but particulate valve metal oxide is not formed and the electrode foils for an electrolytic capacitor of Examples 1 to 10 are compared with each other, the results of comparison show that all of the electrode foils of Examples 1 to 10 are excellent in all of properties, that is, film withstanding voltage, electrostatic capacitance and leakage current.

Thus, according to the present invention, the forming of a boehmite film on the roughened surface of aluminum foil enables the holding ability of particulate gels to be enhanced and particulate valve metal oxide to be formed densely and uniformly in heat treatment as well as plays a role in enhancing the formation efficiency when the anodizing is carried out. Therefore, high electrostatic capacitance can be obtained.

Furthermore, particulate valve metal oxide formed on the boehmite film serves as a buffer layer when the anodizing is carried out, making the growth rate of the dielectric oxide film slow when the formation is carried out. Consequently, it is possible to form a dielectric oxide layer that is thinner, denser and more uniform than a dielectric oxide layer obtained by conventional anodizing. Thus, an electrode foil having small leakage current and high electrostatic capacitance can be obtained by the present invention.

According to the present invention, since an electrode foil with high electrostatic capacitance can be obtained by forming particulate valve metal oxide before anodizing, rated capacity of an electrolytic capacitor using the electrode foil can be enhanced, thus realizing electronic equipment having a smaller size and higher reliability. 

1. A method of manufacturing an electrode foil for an electrolytic capacitor, the method comprising: forming a boehmite film on a roughened surface of an aluminum foil; forming a particulate gel of a valve metal on the surface of the aluminum foil on which the boehmite film is formed; carrying out heat treatment for forming a particulate valve metal oxide from the particulate gel of a valve metal; and anodizing the heat-treated aluminum foil.
 2. The method of manufacturing an electrode foil for an electrolytic capacitor according to claim 1, wherein the anodizing is carried out in such a manner in which the particulate valve metal oxide is covered with an amorphous dielectric oxide film layer.
 3. The method of manufacturing an electrode foil for an electrolytic capacitor according to claim 1, wherein the forming of the boehmite film comprises: dipping the aluminum foil into a weak alkaline aqueous solution; and carrying out heat treatment.
 4. The method of manufacturing an electrode foil for an electrolytic capacitor according to claim 3, wherein the weak alkaline aqueous solution is pH 8 to
 10. 5. The method of manufacturing an electrode foil for an electrolytic capacitor according to claim 1, wherein the forming of the particulate gel of valve metal is dipping the aluminum foil on which the boehmite film is formed into a sol solution containing the valve metal oxide particles having a particle size in a range of 1 to 200 μm.
 6. The method of manufacturing an electrode foil for an electrolytic capacitor according to claim 5, wherein the sol solution is containing metal oxide particles of at least one metal selected from Si, Ti, Zr, Nb, Al and Ta.
 7. An electrode foil for an electrolytic capacitor, comprising: particulate valve metal oxide formed on a roughened surface of an aluminum foil; and an amorphous dielectric oxide film layer covering the particulate valve metal oxide.
 8. The electrode foil for an electrolytic capacitor according to claim 7, wherein the valve metal is at least one metal selected from Si, Ti, Zr, Nb, Al and Ta.
 9. The electrode foil for an electrolytic capacitor according to claim 7, wherein the valve metal oxide is a particle having a particle size in a range of 1 to 200 μm. 